1. Field of the Invention
The present invention relates to a valve for metering fluid.
2. Description of the Related Art
The umbrella term ‘fluid’ for a liquid or flowing medium is used for gases and liquids in conformity with the teaching of hydrodynamics.
Metering valves having piezoelectric actuators make use of the piezoelectric effect of a so-called piezo module. One known piezo module for a piezo-controlled fuel injector (published
German patent application document DE 103 19 599 A1) has a stack of individual piezoelectric elements situated axially one behind the other, which are connected to two contact pins, as well as two cover plates situated at the end faces of the stack; in addition, it has a bourdon tube which engages at the connection plates and generates an axial pressure force on the stack by way of the cover plates in response to the application of an electric voltage at the contact pins. The stack, which is under axial pressure, becomes longer when an electric voltage is applied at the contact pins. The elongation is employed to generate an opening lift of a valve member that controls a metering orifice of the metering valve. The size of the opening lift and the opening time of the valve member define the metered fluid quantity. Such a piezo module, supplemented by an electrical connection to form a piezoelectric actuator, is installed, together with the valve member and a hydraulic coupler, in force-locking manner in a switching force chain of the valve that is acting on the valve member, as described in published German patent application document DE 10 2007 028 490 A1, for example. In order to minimize the tolerances in the metering precision of the valve, it is extremely important that the force relationships in the switching force chain be adjustable in a highly precise manner. This requires a defined initial length of the actuator. However, the length of the piezo module, and thus the length of the actuator, may vary as a result of the piezoelectric effect triggered by temperature fluctuations during storage or by mechanical loading of the actuator during prior work processes; for this reason, to produce the defined initial length of the actuator, the actuator is briefly electrically short-circuited immediately prior to adjusting the switching force chain made up of a valve-closure spring, an actuator and a hydraulic coupler, so that a possibly existing charge voltage is eliminated. Once the electrical connection between the connection element on the actuator and the valve-side plug terminal has been established, the latter is assigned a discharge resistor which is switched parallel to the actuator. The discharge resistor prevents a voltage potential from being applied at the plug terminal of the finished valve, which is disallowed for reasons of safety.